Section 6 The Changing Geography of Your Community Section 6 The Changing Geography of Your Community What Do You See? Learning Outcomes Think About It In this section, you will You have seen how Earth’s lithospheric plates move and change • Use several present-day distributions over time. In some ways, the plates can be compared to gigantic of minerals, rock formations, pieces of a jigsaw puzzle. and fossils to help figure out the distribution of continents. • How would you be able to decide if all the pieces came from the • Construct a map showing the same puzzle? position of continents 250 million • How could you convince someone else that the pieces came years ago by reversing the present from the same puzzle? direction of plate motion. • Recognize a convergence of Record your ideas about these questions in your Geo log. Include presently widely scattered minerals, sketches as necessary. Be prepared to discuss your responses with rock formations, and fossils when all your small group and the class. the continents were part of Pangea. • Describe the context in which the Investigate hypothesis of continental drift was proposed and why it was subject In this Investigate, you will learn about the evidence that supports to criticism. the theory that Earth’s plates have moved and changed over time. • Show that your community has moved through different ecological 1. Begin your work individually. Obtain three copies of the regions over time. diagram on the next page, showing the outlines of the continents at sea level, as well as the boundaries between the continental crust and the oceanic crust. The diagram also shows the locations of rock and mineral deposits, mountain ranges, and plant and animal fossils. Cut out the continents 185 EarthComm EC_Natl_SE_C2.indd 185 7/12/11 1:28:13 PM Chapter 2 Plate Tectonics on the first sheet along the edges of a) Why cut the pieces at the boundaries the continental shelves, which in most between the continental and places are close to the boundaries oceanic crust? between the oceanic lithosphere and b) In which ecological region is your the continental lithosphere. community today: tropics, subtropics, mid-latitudes, subpolar, or polar? 186 EarthComm EC_Natl_SE_C2.indd 186 7/12/11 1:28:13 PM Section 6 The Changing Geography of Your Community c) Coal deposits originated in the a) Draw a border around the map. swamps of tropical forests. Are the b) Sketch in and label the equator and coal deposits shown on the map in latitude lines at 30° and 60° north the tropics today? and south. d) Where do you find mountains c) Title this map “250 Million similar in structure to the Years Ago.” Appalachian Mountains? e) Where do you find rock formations 4. The following diagram shows the similar to those in South America? reconstruction of Pangea that is generally accepted by geologists. Your f) Glossopteris is an extinct seed fern reconstruction is likely to be somewhat that had leaves like ferns of today. different because the evidence you had It produced seeds too large to travel is less detailed. Compare your map by air or float on water. Where are with the following map, and adjust the fossils of these ferns located today? positions of the continents on your map g) Mesosaurus is an extinct freshwater as necessary. reptile that thrived during the Triassic Period (251 to 200 million years ago). Where are fossils of this reptile found today? 2. Rearrange the cut pieces on a blank sheet of paper as the continents now appear and tape them in place. a) Label the outlines “Present.” b) Draw a border around the map. c) Sketch in and label the equator and latitude lines at 30° and 60° north and south. d) Title the map “Present.” 3. Cut out the continents from the second sheet, in the same way as before. Try Generally accepted reconstruction of Pangea, in the Permian Period of geologic time, 250 million years ago. to arrange them on another sheet of paper, as they would have appeared 5. Use your adjusted map, “250 Million 250 million years ago, before the Years Ago,” to answer the following: Atlantic Ocean and the Indian Ocean began to open. You can do this by using a) Which two continents fit two methods: (a) Move each continent together best? in the direction opposite of that shown b) Why do you think the continents do by the arrows on the map of plate not fit together exactly? motions in the Investigate of Section 4; (b) Match similar rock and mineral c) From the map of Pangea, what can deposits, mountain ranges, and fossils you say about the latitude of your from continent to continent. When you community 250 million years ago? have moved all the continents together, tape them in place. 187 EarthComm EC_Natl_SE_C2.indd 187 7/12/11 1:28:14 PM Chapter 2 Plate Tectonics d) In what ecological region was your Arrange them on a new piece of paper, community 250 million years ago? as they might appear 250 million years in the future. You can do so by starting e) Many coal deposits were made more with the present distribution of the than 280 million years ago in the continents and then moving each in tropics. Where were they 250 million the direction shown by the arrows on years ago? Does this make sense? the map of plate motions. Remember Explain your answer. that some plates will be subducted f) Do the Appalachian Mountains line under others. up with other mountain ranges that a) What will fill the spaces between the they resemble? continents in the future? g) Do rock formations in South b) What will happen to the America line up with other Mediterranean Sea? What will be formations that they resemble? created in southern Europe? h) How does Glossopteris appear to c) Where will the southern coast of have migrated to its present fossil California be in 250 million years? distribution, although its seeds could not be carried by the wind or float d) In what latitude and in which on water? ecological region might your community lie in 250 million years? i) How does Mesosaurus appear to have migrated to its present fossil e) How might the change in ecological locations, although it could not swim region affect your community? in the salty ocean? f) Why might your prediction regarding 6. Cut out the continents from the third the future location of your community sheet, in the same way as before. and continent be in error? Digging Deeper DEVELOPMENT OF THE PLATE TECTONICS THEORY In the Investigate, you looked at some of the evidence that supports the idea that the continents of Earth have moved during geologic time. Two Geo Words features of Earth were the subject of intense study in the late 1800s. fossil: any evidence of The first was the discovery of similar fossils on continents that are now past life preserved in separated by oceans. The second was the origin of mountain ranges. Both sediments or rocks. played a part in the early stages of the development of the theory of plate tectonics. In the late 1800s, an Austrian geologist named Eduard Suess (1831–1914) tried to solve a basic geologic question. How do mountain ranges form? He based his model of mountain formation on some of the same principles that you have explored in this chapter. Suess stated that as Earth cooled from a molten state, the most dense materials contracted. These dense materials sank toward the center. The least dense materials “floated” on top. They cooled to form the crust. He then speculated that 188 EarthComm EC_Natl_SE_C2.indd 188 7/12/11 1:28:14 PM Section 6 The Changing Geography of Your Community mountain ranges formed from the contraction and cooling of Earth. He likened this to the way that an apple wrinkles and folds as it dries out and shrinks. Suess went on to explain the origins of oceans and continents. He also explained why there were similar fossils on different continents now separated by oceans. In his model, during the cooling process, parts of Earth sank deeper than others. This is how the ocean basins were formed. Suess claimed that certain parts of the seafloor and continents could rise and sink as they adjusted to changes in the cooling Earth. This led him to propose land bridges between continents. Suess coined the term “Gondwanaland.” This is term he used for a former continent made up of central and southern Antarctica, Africa, Madagascar, and peninsular India. These areas all contained similar fossils, hundreds of millions of years old. Suess thought that the land bridges allowed various animals and plants to migrate and spread without crossing an ocean. Other geologists proposed different models to explain mountains, oceans, and fossils. However, they all generally agreed that Earth’s crust moved up and down. They did not think that it moved very far sideways. Many geologists agreed with the idea of land bridges. It was thought Geo Words that these bridges allowed organisms to move between continents now fold: a bend in a separated by oceans. According to Suess and others, the land bridges planar feature of sank into the ocean long ago. They no longer exist. rock. Not all geologists accepted the theory of a contracting Earth. In 1912, the German geologist Alfred Wegener (1880–1930) proposed the hypothesis of continental drift. He saw a variety of problems with the contraction theory. One problem was the Alps appear to be pressed together. The Alps are a young mountain range.